High-Mobility Ambipolar Benzodifurandione-Based Copolymers with Regular Donor–Acceptor Dyads Synthesized via Aldol Polycondensation

In this study,a family of three benzodifurandione-baseddonor–acceptor(D–A)copolymers,namelypoly[3,7-bis((E)-1-(4-octadecyldocosyl)-2-oxo-6-yl-indolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]difuran-2,6-dionealt-(E)-1,2-di(2,2′-bithiophen-5-yl)ethene](PBDO-DTE),poly[3,7-bis((E)-7-fluoro-1-(4-octadecyldocosyl)-2-oxo-6-yl-indolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]difuran-2,6-dione-alt-(E)-1,2-di(2,2′-bithiophen-5-yl)ethene](PFBDO-DTE),and poly[(3E,7E)-3,7-bis(1-(4-octadecyldocosyl)-2-oxo-6-yl-1,2-dihydro-3Hpyrrolo[2,3-b]pyridin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]difuran-2,6-dione-alt-(E)-1,2-di(2,2′-bithiophen-5-yl)ethene](PNBDO-DTE),was designed and synthesized by employing aldol polycondensation reactions between benzodifurandione and bisindolin-2-ones in high yields.Further incorporation of fluorine or sp2-hybridized nitrogen atoms on the bisindolin-2-one unit induced S…F or S…N nonbonding interactions-locked polymeric conjugated backbones for PFBDO-DTE and PNBDO-DTE,respectively.Ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy measurements revealed that the two copolymers have lower frontier molecular orbitals than that of PBDO-DTE.Atomic force microscopy and two-dimensional grazing-incidence wide-angle X-ray scattering investigations indicated the PNBDO-DTE copolymer could form more ordered molecular aggregation in the solid state than PBDO-DTE and PFBDO-DTE copolymers.We fabricated thin film transistors based on these copolymers on plastic polyethylene terephthalate substrates,and they all showed ambipolar charge transport characteristics.Among them,the PNBDO-DTE-based devices afforded optimal charge transport characteristics with high hole and electron mobilities of 5.16 and 1.33cm^(2)V^(−1)s^(−1),respectively.Our study highlights that aldol polycondensation would be an extremely useful protocol in constructing high-performance polymer semiconductors.

The Potential Game-Changer:A Concept-to-Proof Study on D:A Heterojunction-Free Organic Photovoltaics

Since 1986,the donor-acceptor (D:A) heterojunction has been regarded a necessity for high-efficiency organic photovoltaics (OPVs),due to its unique advantage in compensating the intrinsic limitations of organic semiconductors,such as high exciton binding energy and poor ambipolar charge mobility. While this adversely causes tremendous non-radiative charge recombination and instability issues,which currently become the most critical limits for commercialization of OPVs. Here,we present a concept-to-proof study on the potential of D:A heterojunction free OPV by taking advantage of recent progress of non-fullerene acceptors. First,we demonstrate that the “free carriers” can be spontaneously generated upon illumination in an NFA,i.e.,the 6TIC-4F single layer. Second,the 6TIC-4F layer also exhibits good ambipolar charge transporting property. These exceptional characteristics distinguish it from the traditional organic semiconductors,and relieve it from the reliance of D:A heterojunction to independently work as active layer. As a result,the subsequent OPV by simply sandwiching the 6TIC-4F layer between the cathode and anode yields a considerably high power conversion efficiency ~1%. Moreover,we find the D:A heterojunction free device exhibits two order of magnitude higher electroluminescence quantum efficiency and significantly reduced VOC loss by 0.16 eV compared to those of the D:A BHJ structure,validating its promise for higher efficiency in the future. Therefore,our work demonstrates the possibility of using D:A heterojunction-free device structure for high performance,that can potentially become the next game changer of OPV.